iMac (Retina 5K, 27-inch, 2017) LG monitor flashing off occasionally

Greetings Edw.,

It sounds like there may be a loose connection or a faulty cable if your external display is flashing on and off.

To help isolate the behavior, ensure the cables are connected securely, use a different wall outlet for the power supply, and test with different cables and adapters. Check out additional troubleshooting in the following article: If your external display is dark or low resolution - Apple Support

Check connections

Make sure that your display's power cable and video cable are connected securely and correctly at both ends. Learn more about how to connect a display to your Mac.

If you're using an adapter with your video cable, the adapter must support your display's resolution and refresh rate. Thunderbolt and USB-C adapters must also be compliant with DisplayPort Alt Mode, Thunderbolt 3, or Thunderbolt 4. If you're not sure about your adapter, try a different adapter or replace the video cable with one that doesn't require an adapter.

If the display is connected to a Mac laptop computer with a closed lid, make sure that the Mac is connected to power and using an external keyboard and mouse.

If the issue continues, then it sounds like there may be a hardware issue with the display. To narrow down the issue further, you could attempt to use the display with a different computer. Reach out to the manufacturer if the issue continues with multiple computers, as they'll be best equipped to troubleshoot the display further: Contact Us: Customer Service | LG USA Support

Kind regards.